1. Field of the Invention
The present invention relates to surface-coated tool members of tungsten carbide (WC) based cemented carbide which have hard coatings less susceptible to separation and have superior resistance to wearing and chipping when used as cutting tools for milling or finish turning operations.
2. Prior Art
There is known a surface-coated tool member, which comprises a WC-based cemented carbide substrate and a hard coating formed thereon and comprising one or more layers each composed of one of carbides, nitrides and oxides of metals in groups IVA, VA and VIA of the Periodic Table, solid solutions of these compounds and aluminum oxide.
For example, Japanese Patent Application Laid-Open (18-Month Publication) No. 52-110209 describes a surface-coated WC-based cemented carbide tool member in which the hardness at a portion of the substrate near the surface thereof is reduced 2% to 20% compared with that at a interior portion of the substrate by modifying cobalt (Co) content, titanium carbide (TiC) content and grain size of WC.
Another surface-coated tool member disclosed in Japanese Patent Application Laid-Open No. 54-87719 comprises a soft layer which is formed near the surface of the substrate by subjecting WC-based cemented carbide containing nitrogen to sintering in a vacuum. U.S. Pat. No. 4,610,931 describes a similar tool member.
In each of these tool members, the cobalt content at the portion near the surface of the substrate is more than that at the interior portion thereof, and hence even though the hard coating is subjected to cracking, the cracks are prevented from propagating in the substrate by the tough surface portion containing great cobalt content. Therefore, the tool members exhibit excellent performance particularly in a rough turning operation for steel or cast iron.
However, although the aforesaid tool members are less susceptible to chipping due to their great toughness, the bonding strength between the hard coating and the substrate is not sufficient, and hence the hard coating is susceptible to separation, resulting in abnormal wearing. Accordingly, when a cutting tool composed of the aforesaid prior art tool member is employed in milling operation wherein a great impact is exerted on the hard coating, or in finish turning wherein shear stress is exerted on the hard coating, the tool life is reduced unduly.